Overfill protection for fuel tanks

ABSTRACT

A fuel storage tank is disclosed which is fitted with an overfill protection device for automatically shutting off flow of fuel into the tank when the fuel reaches an intended maximum fill level. The overfill protection device comprises a fill valve located in a fuel fill tube and linked to a diaphragm or piston which is displaceable within a chamber, said chamber communicating with an aperture which is arranged to vent the chamber into the tank. The device may include a float valve for controlling the aperture and maintaining the aperture closed until fuel in the tank reaches a predetermined fill level, whereupon the aperture is opened and the piston or diaphragm is displaced to cause closure of the valve.

FIELD OF THE INVENTION

This invention relates to fuel storage tanks and in particular providesan arrangement for automatically shutting off the incoming flow of fuelwhen the fuel tank is essentially full, thereby preventing overfillingand loss of fuel.

BACKGROUND OF THE INVENTION

In our co-pending UK Application No. 9619580.5, (Publication No.2317382), an overfill protection device (OPD) for fuel tanks isdisclosed. One embodiment described in our co-pending applicationincludes a butterfly or sleeve-type fill valve which is positioned inthe fuel fill line and is controlled by a diaphragm or piston. Air inthe fill line is allowed to bleed into the ullage through a valvecontrolled by a float during a normal filling stage. However, when thetank is almost full, e.g. about 95% full, the bleed line is shut off.This causes pressure to rise in a chamber on one side of the piston ordiaphragm, resulting in movement of the piston or diaphragm whichmovement is transmitted to and closes the fill valve. While theconstructions described in our above application functionsatisfactorily, the arrangement is complicated and requires asignificant number of seals.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an overfill preventionsystem which is less complex and can be operated with no dynamic sealsfor extended operational life.

According to one aspect of the present invention there is provided afuel storage tank having an overfill protection device (OPD) forshutting off flow of fuel into the tank when the amount of fuel in thetank reaches an intended maximum fill level, said OPD including a fillvalve located in or connected to a fuel fill tube and linked to a pistonor diaphragm, which is displaceable in a chamber, said chambercommunicating with an aperture which is arranged to vent the chamberinto the tank when said aperture is open to cause displacement of thepiston or diaphragm and the consequential closure of the fill valve,said aperture being controlled by a vent valve which normally maintainssaid valve closed, but is linked to fuel level sensing means for sensingwhen the level of fuel in the tank reaches said intended maximum filllevel and causing said vent valve to open.

According to one form of the invention there is provided a fuel storagetank having an overfill protection system for shutting off flow of fuelinto the tank when the amount of fuel in the tank reaches an intendedmaximum fill level said overfill protection system including a fillvalve located in a fuel fill tube and linked to a diaphragm or piston,which is displaceable in a chamber, said chamber communicating with anaperture within the ullage above said maximum fill level, and a floatvalve for controlling said aperture and maintaining said aperture closeduntil fuel in the tank reaches a predetermined fill level whereupon theaperture is opened and the piston or diaphragm is displaced to causeclosure of the fill valve.

Fuel storage tanks are commonly replenished by connection to a roadtanker and filled by gravity from the road tanker. As a consequence,during the filling procedure, the pressure in the fill line leading fromthe tanker to the fill or drop tube is greater than the pressure in thetank ullage by an amount approximately equal to the hydrostatic pressureof the tank fuel level above the drop tube exit. When the fill valve isclosed, pressure is equal to the hydrostatic pressure of the road tankerabove the fill valve. The piston is preferably spring-biased with alight spring so that the piston is in its retracted position and thefill valve is normally in the open position. Alternatively, the fillvalve may be separately spring-biased into its closed position but onlyto the extent that it is opened by flow of fuel into the drop tube.

When the float valve opens, the difference in pressure between the fillline and the ullage is sufficient to move the piston and cause closureof the fill valve. The fill valve is preferably a butterfly valve or asleeve valve or other valve which will move easily between its open andclosed position. A butterfly valve meets these requirements becausethere is equal pressure on each side of the valve. However, a poppetvalve may also be employed, especially if it is lightly spring-biasedtowards the closed position.

In contrast with the overfill prevention device (OPD) described in ourabove co-pending application, a piston seal is unnecessary in the OPD ofthe present invention. The annular gap between the piston and cylinderwall acts as a balance orifice with significantly less area than theopen float valve orifice and test valve orifice. When the test valve orfloat valve is opened during filling, a pressure imbalance across thepiston is generated causing the piston to rise against a light springbiasing the butterfly valve into its closed position. When the pistonreaches its full travel it is sealed against the cylinder cap preventingfurther flow through the float valve. Also, it is unnecessary to providean elastomer seal between the butterfly valve and its seat because asmall leak at this point will not matter.

In practice, the fuel will be flowing during normal peak filling atabout 1000 liters per minute and closure of the fill valve will reducethe flow to about 1% or less.

Closure of the fill valve will be immediately apparent to the roadtanker driver, since the sudden closure will give a kick to the deliveryhose and he will then know that the tank is fill and will shut off thesupply valve at the tanker.

An important feature of the system of the invention is that theoperation of the fill valve can be readily tested by providing a testvalve in the line communicating the cylinder with the float valve. Ifthe test valve at this point is open with a positive hydrostaticpressure within the fill line, the fill valve will immediately operate.Also, failure of the interconnecting hose or other conduit between thefill valve and float valve will cause the fill valve to close to providea fail safe condition.

In a modified form of the invention, it is possible to dispense with thefloat valve and instead control the vent valve by means of electricalcontrol means. This arrangement has the advantage that the OPD can belocated entirely within the drop tube or crane T-piece and no otherconnections need be made to the tank. In this form of the invention, theaperture for venting pressure from the chamber may be closed by anelectrically operated vent valve which may be actuated to vent pressureinto the tanks e.g. through a tubular valve stem into the drop tubebelow the fill valve.

During the filling of a fuel tank, it is highly desirable to be able toreduce tank fluid turbulence. The main benefit from reduction ofturbulence is that there will be less disturbance of the heavy fuelvapour layer on top of the liquid fuel, and this will consequentlyreduce vapour line fuel losses.

Normally, the fill tube into a fuel tank terminates just short of thebase of the tank and is cut off at approximately a 45 degree angle, andmay terminate, for example, about 6 inches above the tank base.Turbulence is particularly a problem during the initial stage of thefill cycle when the air in the connecting pipes from the tanker isdriven down the fill tube by the incoming fuel. This initial “slug ofair” rebounds off the tank bottom and produces a large concentration ofair bubbles causing a break up of the normal vapour layer.

Another aspect of the invention seeks to reduce such turbulence byfitting a diffuser to the delivery end of the fill or drop tube.

According to a further aspect of the invention, therefore, there isprovided a fill or drop tube for a fuel storage tank in which thedelivery end of the drop tube is closed by a diffuser having apertureswhich cause fuel emitted from the drop tube to exit therefrom undersubstantially laminar flow conditions.

The exit aperture or apertures from the diffuser are preferably definedby two or more substantially parallel plates arranged substantially atright angles to the longitudinal axis of the drop tube. Conveniently,the apertures are formed from a series of spaced washers attached to theend of the drop tube.

The diffuser of the present invention may be used with any drop tube fora fuel storage tank. It may be used with particular advantage inconjunction with an air transfer valve designed to bleed off airentering the fill line into the ullage. Air transfer valves for thispurpose are described in our co-pending British patent application No.9808483.3. The OPD of the present invention may also be used in fuelstorage tanks in conjunction with an air transfer valve as described inBritish patent application No. 9808483.3.

The diffuser of the invention may also be used in conjunction with theoverfill protection system described above.

Preferred aspects of the present inventions will now be described withreference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of the overfill protection systemfitted into the fill line of a fuel storage tank,

FIG. 2 is a sectional view of a first embodiment of an overfillprotection system in accordance with the invention, showing the fillvalve and operating piston connected to the float and float valve;

FIG. 3 is a section through a second embodiment of an overfillprotection system in accordance with the invention showing the fillvalve, operating piston connected to the float and float valve;

FIG. 4 is a section through a third embodiment of an OPD in accordancewith the invention, in which venting of the chamber above the piston iscontrolled by an electrically actuated valve;

FIG. 5 is a section through a fourth embodiment of an OPD in accordancewith the invention, in which the flow of fuel from a tanker is in-linerather than at right angles to the drop tube;

FIG. 6 is a schematic view of a vehicle storage tank showing the fill ordrop tube during the initial stage of the fill cycle;

FIG. 7 is a view similar to FIG. 6, wherein the conventional fill ordrop tube has been fitted with a diffuser in accordance with theinvention;

FIG. 8 is a section through a preferred embodiment of the diffuser inaccordance with the invention; and

FIG. 9 is a graph showing the pressure changes during filling of thefuel storage tank.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring first to FIG. 1, the overfill prevention system generallyindicated at 1 in FIG. 1, is fitted into the manhole cover 2 of a fueltank 3. The overfill prevention device (OPD.) comprises a fill valve 4,which in the embodiment schematically illustrated is a butterfly valvelocated within the fill or drop tube 5. The fill valve 4 has anoperating mechanism comprising a piston 6 arranged to move in a cylinder7 and a linkage 8 for transmitting movement of the piston to the fillvalve 4. Cylinder 7 is connected by a conduit 9 to a float valve 10,which is normally in the closed position, but is opened when fuelreaches the maximum intended fill level, normally indicated by thebroken line in FIG. 1, normally equivalent to 95% of the contents of thetank.

Conduit 9 also includes a test valve 12 located at a convenient positionoutside the tank. Manual operation of the test valve opens the conduit 9to the ullage space or to atmosphere and enables the operation of thefill valve 4 to be tested. Between the cylinder and float valve 10 islocated a non-return valve 13, arranged to prevent fluid flow from theullage space into the drop tube. Conveniently, this is located adjacentto the cylinder 7.

Piston 6 is preferably spring-biased into the position shown in FIG. 1,e.g. with a light spring 14, so that the piston 6 is retracted and thevalve 4 is normally in the open position. However, the weight of thepiston and associated linkage may provide this function. Alternatively,the flow of fuel into the drop tube may be sufficient to move the valveinto the open position.

Details of the construction of two embodiments of the fill valve andfloat valve are shown in FIGS. 2 and 3. As shown in FIG. 2. the fillvalve, generally indicated at 4 and its associated operating piston 6,can be fitted into the drop tube 5 and the Crane T-junction 15, which isnormally present in conventional fuel storage tanks.

As can be seen from FIG. 2, the fill valve 4 comprises a butterfly valvepivoted at 20 in a tubular fitting 21, which is dimensioned to fitwithin the drop tube 5. Closure of the fill valve 4 is effected byupward movement of a link arm 22, connected by a pivot point 23 to theconnecting rod 24 of piston 6. Piston 6 moves within a cylinder 7 and isbiased into the position shown in FIG. 2 by spring 14. Valve vane 4 isnot a sealing fit in tube 21 since a seal is not necessary for the valveto perform its function and a seal would restrict the movement of thevalve vane 4. As can be seen, the fill valve vane 4 has rounded ends 25which co-operate with a chamfered surface 26 on one end of the tube 21.The pivot axis 20 is preferably aligned with the piston rod 24 to ensuresmooth and rapid closure.

Cylinder 7 communicates with the space in the T-piece 15 via passages 27which are protected with flame-arresting grids 28. A port 29communicates with a conduit 9 leading to the float valve 10. Anon-return valve 13 (indicated schematically in FIG. 1) may be locatedadjacent the port 29 to prevent flow from the ullage into the cylinder7. The non-return valve may, for example, comprise a ball normallyresting in a valve seat, and may be present at any convenient pointbetween the port 29 and the float valve. Air or vapour under pressure isable to lift the non-return valve and pass along conduit 9, but any backpressure which may be developed in the ullage is prevented by thenon-return valve from causing fluid to pass in the opposite direction.

Details of the float valve and the test valve are shown in theright-hand part of FIG. 2.

A float 50 is mounted on the lower end of a tube 51, having an enlargedportion 52 containing apertures 53. Tube 51 slides on an inner tube 54which terminates with a conical solid end 55. Inner tube 51 is providedwith apertures 56, which are normally covered by the tube 51 when thefloat is in the position shown in FIG. 2. Once the level of fuel withinthe tank reaches a position where it begins to lift the float 50, tube51 will be raised on the inner tube 54, and this will effectively openthe communication between aperture 56 and apertures 53. Air, vapour orfuel will then be able to flow from the cylinder 7 along conduit 9, tube54 and out into the ullage of the tank. When the piston reaches the topof its upward travel, its upper rim seats against a seal 75. Thisprevents any vapour or liquid fuel from flowing into the conduit 9.

Mounted above the tube 54 is a mechanism 60 for testing valve 10.Testing mechanism 60 has a port 61 which communicates with the ullage.This port is normally closed by valve 62, but can be opened for testpurposes by depressing stem 63, thereby providing a communicationbetween passage 64 and port 61. Since passage 64 is connected to conduit9, the cylinder 7 will be in communication with the ullage. Valve 62 isspring-biased into its closed position so that once stem 63 is released,the port 61 is then closed.

Testing mechanism 60 comprises a body member 61 in which a bore isformed for receiving an operating rod 62. Rod 62 has a hole 63 at oneend and a threaded extension 64 which carries an abutment 65 fastened bya nut 66. Abutment 65 is positioned in relation to tube 51 so that whenrod 62 is lifted, e.g. by placing a hook in hole 63 and lifting thehook, abutment engages a protrusion 67. This has the effect of manuallyopening the valve 10 and connecting the cylinder 7 with the ullage andthus causing the piston to move upwardly, thereby testing the operationof the fill valve.

Body member 61 also provides a cylindrical block which can be fitted toa tubular port in the top of the tank and a chamber for connecting theconduit 9 and a tube 68, leading to the inner tube 54.

The OPD. operates in the following manner. The road tanker delivery pipewill be connected to incoming branch conduit 16, and fuel will then bedelivered to the tank 3 via the drop tube 5. Because of the hydrostaticpressure of fuel delivered from the tanker, the pressure within the droptube 5 and the Crane piece 15 will be higher than that within the ullageof the tank. Air, vapour or fuel will be able to leak past the piston 7and pressurise the line 9, but no movement of the piston will take placebecause the conduit 9 is closed at the other end by the float valve 10.

When the tank reaches about 95% of its filled capacity, float 11 willbegin to rise and this will open valve 10. The pressure in conduit 9will suddenly drop and pressure within the fill line will cause piston 6to suddenly rise and close the valve 4. The tanker driver will notice asharp kick to the delivery hose, which will then indicate to him thatthe tank is full, and flow into the tank will essentially stop.

Prior to the delivery, proper operation of the OPD. can be checked bylifting the rod 62, and this will immediately trigger closure of thevalve 4.

FIG. 2 shows in dotted lines the position of the vane of the valve 4 inits closed position. Mounted below the vane 4 is a compensator valve 70.This consists of a pair of flaps 71 pivotally mounted on a pivot axis 72and spring-loaded by a light spring into a closed position, as shown infull lines in FIG. 2. The flaps 71 open under the flow of fuel into thedrop tube (as indicated by the broken lines) and ensure that there is apositive pressure in the drop tube.

It will be appreciated that the crane T-piece 15 and the conduit 9 areconveniently located within an inspection pit which is normallyprotected by a cover plate not shown. For inspection purposes, the coverplate is removed for testing the operation of the OPD by lifting rod 62.When no fuel is being introduced into the storage tank, the movement ofthe piston and valve can be directly tested using a rod introduciblethrough the port 29. For this purpose, the port may be accessed byremoving a nut 73 and attaching one end of a threaded rod to a threadedsocket 74 in the end of piston 24, which can then be manually lifted totest the movement of the piston 6 and valve 4.

FIG. 3 shows a second embodiment which essentially differs from thatshown in FIG. 2 by utilising a poppet valve as the fill valve 4, and thesame reference numerals are used to show equivalent parts. In thisembodiment, valve 4 comprises a valve seat 80 which is received within atubular support 81 located at the upper end of a drop tube 5. A poppetvalve 82 is slidably mounted on a valve stem 83 and urged into itsclosed position by a light spring 84. Valve stem 83 also constitutes atits upper end a piston rod and carries piston 6 attached to its upperend. Rod 83 is slidably supported in a block 85 fixed to the base ofcylinder 7. The assembly comprising cylinder 7 and block 85 is fixedinto the top of T-piece 15. Block 85 has one or more apertures 27 whichare protected with flame-arresting grids 28.

The embodiment of FIG. 3 operates in a similar way to the embodiment ofFIG. 2. When fuel delivery pipe 16 is connected to a road tanker andfuel is delivered, the poppet valve opens by sliding downwardly on thestem 83 and compressing the spring 84 until the lower face of valvemember 82 rests on abutment 85. When the level of fuel in the tankreaches the intended maximum level, the float is raised and opens thevalve 10. This causes the piston to move suddenly upwardly in thecylinder 7, lifting the valve member 82 and closing it against its seat80. In a similar way to the embodiment of FIG. 2, this sudden closure ofthe valve 4 imparts a “kick” to the tanker delivery hose and shuts offthe main flow of fuel.

Rapid operation of the piston is ensured by proper selection of therelative areas of the piston and the poppet valve. Thus, the area of thepiston should be significantly larger than that of the poppet valve,e.g. twice as large. Also, the total area of the aperture in the floatvalve should be large in order to ensure rapid reduction of pressure inthe conduit 9 when the float valve opens. In the case of bothembodiments, it is important that the area of the gap between the pistonand the cylinder should be small, compared with the area of the orificein the float valve and also small compared with the area of the ports27. This has the effect of restricting flow between the piston andcylinder and assists rapid response when the float valve is opened. Itwill be appreciated that the level at which the float valve operates canbe adjusted by providing tubes 68 and 54 with threads so that byrotating tube 54, the float valve assembly can be raised or lowered withrespect to the block 61. Also, the position of abutment 65 on threadedrod 64 can be similarly raised or lowered.

FIG. 4 shows a section through a third embodiment of an OPD inaccordance with the invention. The construction of the OPD in thisembodiment is similar to that shown in FIG. 3 and the same referencenumerals are used to indicate equivalent components. The majordifference is that in the embodiment of FIG. 4, the chamber 7 is notconnected to a float valve by an external conduit 9. Instead, thechamber 7 communicates with an aperture 201 formed at the top of tubularvalve stem 283. Tubular stem 283 is connected to tubular extension guide284 which carries at its lower end an abutment 85. Poppet valve 82 iscarried by stem 284 and it operates in the manner described above inrelation to FIG. 3. Aperture 201 is normally closed by valve head 202which is spring loaded into its closed position. Mounted above thechamber 7 and encircling the stem of valve 202 is an electro-mechanicalcoil 203 which, when energised with an electrical current, raises valve202 and thus opens aperture 201. Coil 203 is an intrinsically safe DCcoil to BS 5501 standard. Lifting of valve stem 202 causes aperture 201to open and pressurised fluid in chamber 7 to be vented through thevalve stem 284 and out through exit 205 on the tank side of the poppetvalve 82. This causes the poppet valve to be lifted onto seat 80, thusshutting off the flow of fuel and indicating to the tanker driver thatthe storage tank is full or nearly so.

Coil 203 may be energised by an electrical signal from an electronicsensor which senses (e.g. by capacitance measurements) when the tank isalmost full.

A likely sequence of operation would be for the coil 203 to be energisedin this way when the tank is 95% full. After, say, a timed delay of 30seconds, the electronic tank gauging equipment may be arranged tode-energise the coil which would permit rapid draining down of the tankcontents, after the tanker driver has shut off the delivery valve at thetanker.

The electronic gauging equipment may be also arranged to energise thecoil again if the tank level exceeds 97% of the fill level and, perhaps,cause an alarm to be activated. In this case, it may be arranged thatthe hose can be drained down only via a manual isolator switch.

In FIG. 4, satisfactory operation of the OPD may be tested by liftingthe valve stem 202 by means of the button 204.

FIG. 5 shows a further modification of the embodiment of FIG. 3, whichis adapted for tanks having an in-line fill pipe from the tanker. Theconstruction of the fill valve and operating piston is the same as inFIG. 3, and the same reference numerals are given for equivalent partsand components. Instead of the T-piece 15, the fill valve and piston arehoused in an adapted globe valve casing 301. Fuel from the tanker entersthrough port 302, flows through the fill valve and out through an exitport 303, which is connected to the upper end of the storage tank droptube. Operation of the fill valve may be by means of a connection to afloat valve-controlled aperture through port 29 or by means of anelectrically energised coil as in the embodiment of FIG. 4.

Although the embodiments of OPD's illustrated above have been describedin relation to gravity fed fuel storage tanks, it is also possible tofill tanks fitted with such OPD's by a pumped fuel supply and in suchcases, the tanks may be above ground.

The operation of the diffuser will now be described with references toFIGS. 4 to 6.

Referring to FIG. 6, this shows a fuel tank 100 fitted with a standarddrop tube 102 and crane junction 103. Fuel is delivered from a roadtanker via an inlet conduit 104 and passes down the drop tube 102.Conventional drop tubes as shown are cut at an angle of about 45degrees, so that liquid emanating from the drop tube will tend to strikethe bottom of the tank obliquely. In the initial stage of the deliveryof fuel, the air initially present in the delivery hose and inlet line104 will be compressed by the advancing fuel front and this will causethe generation of large air bubbles as illustrated pictorially in FIG.7. These bubbles and pockets of air rebound off the tank base and breakup the top surface of the liquid in the tank, disrupting the normalvapour level and generating large amounts of additional vapour.

The vapour so produced is vented from the tank via the vent line 105. Atthe same time, the air and liquid moving at relatively high velocity outof the drop tube 102 will disturb sediment at the bottom of the tank,causing this to be suspended in the fuel. Such sediment can causedifficulties not only for vehicles supplied from the tank, but in theoperation of the valves of the filling station itself.

Referring to FIG. 7, this shows the storage tank of FIG. 6 with the droptube modified and fitted with a diffuser in accordance with theinvention. Diffuser 106 is fitted to the end of the drop tube which iscut off at right angles to its axis rather than at an angle as in FIG.7. The characteristic of the diffuser 106 is that it includes a closedoff lower end 108 and a series of horizontal vanes 111, which promotelaminar flow of fuel and air substantially at right angles to the axisof the drop tube. Preferably, the vanes extend around substantially thewhole of the circumference of the diffuser, and the effect of the vanesis to break up the air pockets into smaller bubbles, and cause muchreduced disturbance.

The details of the construction of the diffuser are shown in FIG. 8.

The diffuser comprises a tubular part 107 attached to the lower end ofthe drop tube 102, and having a base 108 which carries a cone-shapeddeflector 109. A series of slot-shaped apertures 110 are formed by aseries of vanes 111, which are mounted on studs 112, screwed into thetubular part 107. Spacers 113 between the vanes 111 control the size ofthe slots 110.

FIG. 9 is a graph showing the pressure changes which occur within theT-piece 15 at various times after fuel delivery is commenced to a fuelstorage tank fitted with an OPD in accordance with the invention.

At the start of the tank filing operation, air in the pipework leadingto the T-piece 15 from the road tanker is compressed ahead of theadvancing fuel slug. This is depicted as the area A in the graph duringwhich the pressure in the T-piece commonly reaches a maximum of about0.1 bar.

Approximately 20 seconds after the tanker has started delivering fuel,the compressed bubble or slug of air is displaced through the drop tubeinto the tank—see FIG. 4. As a result the pressure falls sharply asshown at point B. The pressure falls to a negative pressure of about−0.1 bar because of the siphon effect of fuel passing down the droptube. As the tank fills the siphon effect is reduced and the pressuregradually rises along line C. At point D, the pressure changes topositive as the resistance pressure drop exceeds the diminished siphoneffect.

As the tank approaches its full condition, the pressure rises to above30 mbar. This is indicated by the shaded area E which is the area inwhich the OPD operates.

Point F indicates the usual finish of delivery of fuel below the safeworking capacity of the tank.

Point G represents 95% of tank capacity. At this point, the OPD isactuated and the flow inertia causes a sharp pressure spike as the fillvalve closes (point H), giving a visible kick to the tanker deliveryhose.

Pressure then falls to the hydrostatic head difference between thetanker fuel level and the OPD. This is typically about 150 mbar and thepressure at the OPD remains at this point until the taker delivery valveis closed, whereupon the fill valve resets because the pressure in thefill line 16 falls and the spring 14 or 84 returns the piston to itsoriginal position.

What is claimed is:
 1. A fuel storage tank (3) having an overfillprotection device (OPD) for shutting off flow of fuel into the tank whenthe amount of fuel in the tank reaches an intended maximum fill level,said OPD including a fill valve (4) located in or connected to a fuelfill tube (5) and linked to a piston or diaphragm (6), which isdisplaceable in a chamber, said chamber communicating with an aperture(53) which is arranged to vent the chamber into the tank when saidaperture is open to cause displacement of the piston or diaphragm andthe consequential closure of the fill valve, said aperture beingcontrolled by a vent valve which normally maintains said valve closed,but is linked to fuel level sensing means for sensing when the level offuel in the tank reaches said intended maximum fill level and causingsaid vent valve to open.
 2. A storage tank as claimed in claim 1 whereinsaid vent valve is a float valve (10) which is arranged to maintain saidaperture closed until fuel in the tank reaches said intended maximumfill level, whereupon fuel in said tank actuates said float valve tocause the aperture to open and the piston to be displaced in thechamber.
 3. A storage tank as claimed in claim 1 wherein the vent valveis electrically operated and is actuated by an electrical signal fromsaid sensor when fuel in the tank reaches said intended maximum filllevel.
 4. A storage tank as claimed in 1 wherein the fill valve islinked to a piston which is movable in a cylinder.
 5. A storage tank asclaimed in claim 3 wherein the cylinder communicates with a tube whichextends into an ullage space and carries a float which is movable inresponse to rising fuel level to open the aperture.
 6. A storage tank asclaimed in claim 5 in which the chamber communicates with the ullage viaa conduit and said conduit includes a test valve which is normallyclosed, but on operation opens the conduit to the ullage and therebycloses the fill valve.
 7. A storage tank as claimed in claim 1 whereinthe fill valve is a butterfly valve.
 8. A storage tank as claimed inclaim 1 wherein the fill valve is a poppet valve.
 9. A storage tank asclaimed in claim 1 wherein the fuel fill tube has a lower open end whichis fitted with a diffuser, said diffuser having apertures which causefuel emitted from the fill tube to exit therefrom under substantiallylaminar flow conditions.
 10. A storage tank as claimed in claim 9wherein the diffuser comprises two or more substantially parallel platesdisposed substantially at right angles to the longitudinal axis of thedrop tube.